Start with a real reference
Choose a T-shirt whose fit you enjoy after it has been washed and worn. Try it on first: decide whether you want the new tee to be wider, longer or about the same. A size label alone cannot describe those differences. Lay your reference flat without stretching it and smooth out obvious folds. You only need a flexible measuring tape and somewhere to record the numbers.
Measure the width across the chest
Measure straight across the front from one underarm seam to the other. Record this as the flat chest width, with the unit clearly marked. A size chart may show either a flat width or a full circumference; these are different measurements. Read the chart's labels rather than comparing the numbers automatically. If its measuring method is unclear, ask before ordering.
Check length and shoulders separately
Measure the length using the same start and end points shown in the product's size guide. Some guides measure from the highest shoulder point, others from a different position. Note where your reference tee finishes when you wear it. For shoulder width, compare the shoulder seams only when the garment construction and chart make that comparison useful; a dropped shoulder is part of the design of many oversized tees.
Compare the shape, not just the label
Write down your reference measurements beside the product chart. A tee can be wide enough and still longer than you want. Decide which measurement matters most to you, then check the others. Avoid automatically sizing up an already oversized cut. If you are between sizes, think about your preferred silhouette and whether you plan to layer underneath.
Do a final movement check
Sit down, raise your arms and wear the reference tee with your usual trousers. This helps you describe the fit you actually need. Keep your measurements on your phone for next time. Browse the T-shirt collection, check the individual size guide, and read our oversized fit guide for more context.